JP2020200460A - Polyvinyl alcohol, method for producing the same, and use of the same - Google Patents

Abstract

To provide PVA which can suppress formation of coarse particles in a vinyl-based resin obtained when used as a dispersion stabilizer for suspension polymerization of a vinyl compound and can obtain particles having uniform diameters, and can suppress occurrence of fish eyes, and to provide PVA capable of simply obtaining a crosslinked body having excellent water resistance.SOLUTION: Polyvinyl alcohol has a degree of saponification of 70 mol% or more and less than 99.9 mol%, has a degree of viscosity average polymerization of 400 or more and less than 1,800, contains 0.05 mol% or more and less than 0.5 mol% of an aldehyde group at its terminal, and has an absorbance at 280 nm of 0.1 mass% aqueous solution of 0.17 or more and less than 0.55.SELECTED DRAWING: None

Description

The present invention relates to polyvinyl alcohol having a saponification degree and a viscosity average degree of polymerization in a specific range, further containing a specific amount of an aldehyde group at the terminal, and having a specific absorbance. The present invention also relates to a crosslinked product using the polyvinyl alcohol, a dispersion stabilizer for suspension polymerization of a vinyl compound, and a method for producing a vinyl resin.

Polyvinyl alcohol having a reactive functional group (hereinafter, polyvinyl alcohol may be abbreviated as "PVA") has conventionally been a suspension of an adhesive, a paper coating agent, a polarizing film, and a vinyl compound (for example, vinyl chloride). It is used in various products such as dispersion stabilizers for turbid polymerization. In particular, the cross-linked polymer synthesized by the cross-linking reaction between the reaction active site and the cross-linking agent is usually stronger, heat-resistant, and resistant than the same type of linear polymer because the movement of the molecular chain is restricted in the three-dimensional direction. Its usefulness is high because it has excellent solvent properties and particularly excellent water resistance. Further, as a major use of PVA, there is a dispersion stabilizer for suspension polymerization of vinyl compounds, and various PVAs are used.

As a modified PVA having a reactive functional group having particularly high cross-linking performance, a modified PVA having a methylene hydrogen structure sandwiched between two carbonyl groups typified by an acetoacetyl group can be mentioned (Patent Document 1).

Further, in Patent Documents 2 and 3, the heat-treated PVA is dispersed for suspension polymerization of the vinyl compound for the purpose of improving the polymerization stability at the time of polymerization of the vinyl compound and suppressing the coarsening of the obtained vinyl-based polymer. It is used as a stabilizer. In the present specification, the term "polymerization stability" means that the dispersibility of droplets made of a vinyl compound is good at the time of polymerization, and as a result, particles of a vinyl-based polymer having a uniform diameter with suppressed coarse graining are obtained. Means to be.

Japanese Unexamined Patent Publication No. 2014-205826 Japanese Unexamined Patent Publication No. 51-45189 Japanese Unexamined Patent Publication No. 2004-250695

However, the diketene used in Patent Document 1 is highly toxic to living organisms, and there is a risk of explosion when air and steam are mixed. Therefore, a method for synthesizing modified PVA having a reactive functional group more safely has been developed. I was asked. Further, in Patent Document 1, it is necessary to use a special cross-linking agent, and it is difficult to easily obtain a cross-linked product having excellent water resistance by using an acid or the like.

Further, when the modified PVA described in Patent Documents 2 or 3 is used as the dispersion stabilizer for suspension polymerization, although the polymerization stability at the time of polymerizing the vinyl compound is improved, the obtained vinyl-based polymer contains fine powder. It was insufficient to meet the demands of recent years, such as many. Furthermore, there are still problems such as many fish eyes due to bumps and defects when the vinyl polymer is made into a sheet.

The present invention has been made to solve the above problems, and when used as a dispersion stabilizer for suspension polymerization of a vinyl compound, it is possible to suppress the formation of coarse particles in the obtained vinyl resin and the diameter. It is an object of the present invention to provide a PVA capable of obtaining uniform particles and suppressing the occurrence of fish eyes. Another object of the present invention is to provide a PVA capable of easily obtaining a crosslinked product having excellent water resistance.

The present invention has completed the present invention by finding that PVA having a saponification degree and a viscosity average degree of polymerization in a specific range, a specific amount of an aldehyde group at the terminal, and a specific absorbance solves the above problems. ..

That is, the above problems are that the degree of saponification is 70 mol% or more and less than 99.9 mol%, the viscosity average degree of polymerization is 400 or more and less than 1800, and the terminal contains 0.05 mol% or more and less than 0.5 mol% of aldehyde groups. However, it is solved by providing polyvinyl alcohol having an absorbance of a 0.1% by mass aqueous solution at 280 nm of 0.17 or more and less than 0.55.

At this time, it is preferable to have a structure represented by the following formula (1) at the end.

（式（１）中、Ｘは単結合、置換基を有してもよいアルキレン基又は置換基を有してもよいアリーレン基を表し、＊は結合手を表す。）

(In the formula (1), X represents a single bond, an alkylene group which may have a substituent or an arylene group which may have a substituent, and * represents a bond.)

It is also preferable that X is an alkylene group having 1 to 6 carbon atoms.

At this time, a preferable method for producing the above-mentioned polyvinyl alcohol is to polymerize the vinyl ester in the presence of dialdehyde or trialdehyde to obtain a polyvinyl ester, and then saponify the polyvinyl ester.

Further, a dispersion stabilizer for suspension polymerization of a vinyl compound containing the above polyvinyl alcohol is a preferred embodiment of the present invention. A method for producing a vinyl-based resin in which a vinyl compound is suspended and polymerized in the presence of the above-mentioned polyvinyl alcohol is also a preferred embodiment of the present invention.

Further, the present invention is a crosslinked product obtained by cross-linking the polyvinyl alcohol and having an elution rate of less than 10% when a film having a thickness of 100 μm made of the crosslinked product is immersed in hot water at 80 ° C. for 1 hour. Is a preferred embodiment of. At this time, cross-linking the polyvinyl alcohol in the presence of an acid catalyst is a preferable method for producing the cross-linked product.

When the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, it exhibits high polymerization stability, so that it is possible to suppress the formation of coarse particles in the obtained vinyl resin and the particles have a uniform diameter. Can be obtained, and the occurrence of fish eyes can be suppressed. Further, by using the PVA of the present invention, a crosslinked product having excellent water resistance can be easily obtained.

[Polyvinyl alcohol]
The polyvinyl alcohol of the present invention has a saponification degree of 70 mol% or more and less than 99.9 mol%, a viscosity average degree of polymerization of 400 or more and less than 1800, and an aldehyde group at the terminal of 0.05 mol% or more and less than 0.5 mol%. It is characterized by having a 0.1% by mass aqueous solution having an absorbance at 280 nm of 0.17 or more and less than 0.55.

It is important that the degree of saponification of PVA is 70 mol% or more and less than 99.9 mol%. When PVA having a saponification degree of less than 70 mol% is used as a raw material for the crosslinked product, the water resistance of the obtained crosslinked product is lowered. When used as a raw material for a crosslinked product, the saponification degree of PVA is preferably 80 mol% or more, and more preferably 90 mol% or more. On the other hand, it is difficult to produce PVA having a saponification degree of 99.9 mol% or more.

Further, when PVA having a saponification degree of less than 70 mol% is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is lowered, and the obtained vinyl resin has more coarse particles and a uniform diameter. No particles can be obtained. In addition, a large number of fish eyes are generated in the obtained vinyl resin. On the other hand, it is difficult to produce PVA having a saponification degree of 99.9 mol% or more. When used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the saponification degree of PVA is preferably less than 90 mol%, more preferably less than 85 mol%, still more preferably less than 80 mol%. .. The saponification degree is a value obtained by measuring according to JIS K 6726: 1994.

It is important that the viscosity average degree of polymerization of PVA (hereinafter, may be abbreviated as the degree of polymerization) is 400 or more and less than 1800. When PVA having a degree of polymerization of less than 400 is used as a raw material for the crosslinked product, the water resistance of the obtained crosslinked product is lowered. When used as a raw material for a crosslinked product, the degree of polymerization of PVA is preferably 550 or more. On the other hand, when the degree of polymerization is 1800 or more, the productivity of PVA decreases. Further, when PVA having a degree of polymerization of 1800 or more is used as a raw material for the crosslinked product, the viscosity of the liquid becomes too high and the handleability deteriorates. The degree of polymerization of PVA is preferably less than 1600.

Further, when PVA having a degree of polymerization of less than 400 is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is lowered, and in the obtained vinyl-based resin, coarse particles are increased and particles having a uniform diameter are produced. I can't get it. The degree of polymerization of PVA is preferably 550 or more. On the other hand, when the degree of polymerization is 1800 or more, the productivity of PVA decreases. Further, when PVA having a degree of polymerization of 1800 or more is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is lowered, and in the obtained vinyl-based resin, coarse particles are increased and particles having a uniform diameter are produced. I can't get it. In addition, a large number of fish eyes are generated in the obtained vinyl resin. When used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the degree of polymerization of PVA is preferably less than 1500, more preferably less than 1300, and even more preferably less than 1000.

The viscosity average degree of polymerization is a value obtained by measuring according to JIS K 6726: 1994. Specifically, when the degree of saponification is less than 99.5 mol%, the PVA saponified to a degree of saponification of 99.5 mol% or more is subjected to the ultimate viscosity [η] (L / L /) measured at 30 ° C. in water. The viscosity average degree of polymerization (P) is determined by the following formula using g).
P = ([η] × 10 ^{4 /} 8.29) ^{(1 / 0.62)}

It is important that the PVA of the present invention contains 0.05 mol% or more and less than 0.5 mol% of an aldehyde group at the terminal. The aldehyde group is a reactive functional group and can be used for a cross-linking reaction by acetalization with a hydroxyl group, a cross-linking by a radical reaction, or the like. Further, since PVA having an aldehyde group has a high adsorptive power to a vinyl compound, the polymerization is stable when it is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, and coarse particles and fine powder are formed in the obtained vinyl resin. Is reduced. Furthermore, the occurrence of fish eyes can be reduced when the obtained vinyl resin is made into a sheet.

When PVA having an aldehyde group content of less than 0.05 mol% is used as a raw material for the crosslinked product, the water resistance of the obtained crosslinked product is lowered. The content of the aldehyde group is preferably 0.08 mol% or more. On the other hand, PVA having an aldehyde group content of 0.5 mol% or more has low productivity. Further, when such PVA is used as a raw material for a crosslinked product, the viscosity of the liquid becomes too high and the handleability deteriorates. The content of the aldehyde group is preferably less than 0.45 mol%, more preferably less than 0.4 mol%.

Further, when PVA having an aldehyde group content of less than 0.05 mol% is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is lowered and coarse particles are increased in the obtained vinyl resin. At the same time, particles having a uniform diameter cannot be obtained. In addition, a large number of fish eyes are generated in the obtained vinyl resin. The content of the aldehyde group is preferably 0.08 mol% or more. On the other hand, PVA having an aldehyde group content of 0.5 mol% or more has low productivity. Further, when such PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is lowered, and coarse particles are increased in the obtained vinyl resin, and particles having a uniform diameter can be obtained. Can not. In addition, a large number of fish eyes are generated in the obtained vinyl resin. The content of the aldehyde group is preferably less than 0.45 mol%, more preferably less than 0.4 mol%. The content of the aldehyde group can be determined from the ¹ H-NMR spectrum of the vinyl ester polymer before saponification in the deuterated chloroform solvent and the ¹ H-NMR spectrum of PVA in heavy DMSO or heavy water solvent.

The PVA of the present invention may contain a functional group other than an aldehyde group, but the content thereof is preferably less than 5 mol%, more preferably less than 1 mol%, and 0.1 mol%. It is more preferably less than.

It is important that the absorbance of a 0.1 mass% aqueous solution of PVA at 280 nm is 0.17 or more and less than 0.55. When PVA having an absorbance of less than 0.17 is used as a raw material for the crosslinked product, the water resistance of the obtained crosslinked product is lowered. The absorbance is preferably 0.21 or more, more preferably 0.24 or more, and even more preferably 0.28 or more. On the other hand, PVA having an absorbance of 0.55 or more has low productivity. Further, when such PVA is used as a raw material for a crosslinked product, the viscosity of the liquid becomes too high and the handleability deteriorates. The absorbance is preferably less than 0.52, more preferably less than 0.48, and even more preferably less than 0.45.

Further, when PVA having an absorbance of less than 0.17 is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is lowered, and the obtained vinyl resin has more coarse particles and a uniform diameter. No particles can be obtained. In addition, a large number of fish eyes are generated in the obtained vinyl resin. The absorbance is preferably 0.21 or more, more preferably 0.24 or more, and even more preferably 0.28 or more. On the other hand, PVA having an absorbance of 0.55 or more has low productivity. Further, when such PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is lowered, and coarse particles are increased in the obtained vinyl resin, and particles having a uniform diameter can be obtained. Can not. In addition, a large number of fish eyes are generated in the obtained vinyl resin. The absorbance is preferably less than 0.52, more preferably less than 0.48, and even more preferably less than 0.45.

The above absorbance indicates the amount or chain of ethylenic double bonds present in the main chain of PVA. When the absorbance is within the above range, the adsorptivity between PVA and the vinyl compound is improved, and when it is used as a dispersion stabilizer for suspension polymerization of the vinyl compound due to the synergistic effect with the aldehyde group at the terminal of PVA. The polymerization stability of is further improved. When PVA having an absorbance within the above range is used as a raw material for the crosslinked product, the water resistance of the obtained crosslinked product is improved. The absorption at a wavelength of 280 nm is derived from the structure of [-CO- (CH = CH) _2- ] in PVA. The introduction of this structure can be achieved by using aldehydes as a denaturing agent, or by using a monomer or the like that can introduce an ethylenic double bond into the main chain by copolymerization. When the absorbance is out of the above range, the adsorptive power between PVA and the vinyl compound is reduced, or the PVA and the vinyl compound are in a dissolved state. Therefore, the effect as a dispersion stabilizer for suspension polymerization cannot be exhibited. Further, when PVA having an absorbance outside the above range is used as a raw material for the crosslinked product, the water resistance of the obtained crosslinked product is lowered. The conditions of the absorbance measuring device and the like are as described in Examples described later.

The PVA of the present invention preferably has a structure represented by the following formula (1) at the end.

（式（１）中、Ｘは単結合、置換基を有してもよいアルキレン基又は置換基を有してもよいアリーレン基を表し、＊は結合手を表す。）

(In the formula (1), X represents a single bond, an alkylene group which may have a substituent or an arylene group which may have a substituent, and * represents a bond.)

In formula (1), X is a single bond, an alkylene group which may have a substituent or an arylene group which may have a substituent. The carbon number of X is preferably 0 to 8, more preferably 1 to 6, further preferably 1 to 4, particularly preferably 2 to 4, and 3 to 4. Is the most preferable. X is preferably an alkylene group or an arylene group having the above carbon number, and more preferably an alkylene group having the above carbon number. When X satisfies the above aspect, the introduction of the aldehyde group of PVA is easy, the water solubility of the obtained PVA is good, and the polymerization stability when PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound. Is good, or the obtained vinyl-based resin has a tendency to suppress the occurrence of fish eyes.

Examples of the substituent that the alkylene group or arylene group may have include an alkyl group, an aryl group, a hydroxy group, an aldehyde group, a carboxy group, an amino group and the like.

(Manufacturing method of PVA)
The method for producing PVA of the present invention is not particularly limited, but a suitable production method is a method in which a vinyl ester is polymerized in the presence of dialdehyde or trialdehyde to obtain a polyvinyl ester, and then the polyvinyl ester is saponified. ..

Examples of the polymerization method include known methods such as a massive polymerization method, a solution polymerization method, a suspension polymerization method, an emulsion polymerization method, and a dispersion polymerization method. From an industrial point of view, a solution polymerization method, an emulsion polymerization method, and a dispersion polymerization method are used. Is preferable. In the polymerization operation, any of the batch method, the semi-batch method and the continuous method can be adopted.

Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl caprylate, vinyl versatic acid and the like, and vinyl acetate is particularly preferable from an industrial point of view.

The type of dialdehyde or trialdehyde is not particularly limited, and examples thereof include glyoxal, malondialdehyde, succinaldehyde, glutaraldehyde, adipaldehyde, heptandial, octanedial, nonandial, and benzenetricarbaldehyde. Among them, carbon is easy to introduce an aldehyde group, has good water solubility of the obtained PVA, and has good polymerization stability when PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound. A dialdehyde or trialdehyde having a number of 2 to 10 is preferably used, and a dialdehyde having 2 to 6 carbon atoms is more preferably used. Glutaraldehyde is more preferably used from the viewpoint of being easily available and capable of suppressing the generation of fish eyes in the obtained vinyl resin. The amount of dialdehyde or trialdehyde used is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, based on 100 parts by mass of vinyl ester. On the other hand, the amount of dialdehyde or trialdehyde used is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less, based on 100 parts by mass of vinyl ester. One type of dialdehyde or trialdehyde may be used alone, or two or more types may be used in combination.

It is preferable that a solvent is used in the polymerization step and the mass ratio of the vinyl ester to the solvent is vinyl ester / solvent = 100/0 to 90/10. If the mass ratio of the vinyl ester to the solvent exceeds 90/10, the performance as a dispersion stabilizer tends to deteriorate when PVA is used as a dispersion stabilizer for suspension polymerization of a vinyl compound.

In the polymerization step, a monomer other than vinyl ester may be copolymerized as long as the gist of the present invention is not impaired. By copolymerizing another monomer with a vinyl ester, the structure of another monomer unit can be contained in the main chain of the obtained polymer. Examples of the other monomer include α-olefins such as ethylene and propylene; (meth) acrylic acid and salts thereof; methyl (meth) acrylic acid, ethyl (meth) acrylic acid, n (meth) acrylic acid. -Propyl, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i-butyl (meth) acrylate, t-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, (meth) ) (Meta) acrylic acid esters such as dodecyl acrylate and octadecyl (meth) acrylate; (meth) acrylamide; N-methyl (meth) acrylamide, N-ethyl (meth) acrylamide, N, N-dimethyl (meth) (Meta) of acrylamide, diacetone (meth) acrylamide, (meth) acrylamide propanesulfonic acid and its salts, (meth) acrylamide propyldimethylamine and its salts or quaternary salts thereof, N-methylol (meth) acrylamide and its derivatives, etc. Acrylamide derivatives; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether; acrylic acid nitrile, methacrylonitrile, etc. Butyls; vinyl halides such as vinyl chloride and vinyl fluoride; vinylidene halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid, itaconic acid, fumaric acid and the like. Examples thereof include unsaturated dicarboxylic acids and salts thereof or esters thereof; vinylsilyl compounds such as vinyltrimethoxysilane; isopropenyl acetate and the like. When copolymerizing such other monomers, the content thereof is usually 5 mol% or less.

As the solvent used in the polymerization step, an alcohol solvent is preferable. Examples of the alcohol solvent include methanol, ethanol, propanol and the like, and methanol is preferable. These can be used alone or in combination of two or more.

In the production method of the present invention, a method of polymerizing a vinyl ester in the presence of dialdehyde or trialdehyde and water to obtain a polyvinyl ester and then saponifying the polyvinyl ester is more preferable. The mass of water at this time is preferably 0.3 times or more, more preferably 0.4 times or more the amount of dialdehyde or trialdehyde. On the other hand, the mass of water is preferably less than 9 times, more preferably less than 4 times, and even more preferably less than 2 times the amount of dialdehyde or trialdehyde.

Normally, the addition of water reduces the productivity of the polymerization step and the subsequent saponification step, but in the present invention, it has been found that the addition of water makes it possible to easily introduce an aldehyde structure at the terminal. The reason for this is not clear, but the addition of water suppresses the side reactions of dialdehyde or trialdehyde (hemiacetal, acetalization, cyclization) in the polymerization process, and the chemical equilibrium is higher than that of the product due to the side reaction. It is presumed that this is because the reaction between dialdehyde or trialdehyde and vinyl ester proceeds efficiently by leaning toward the state of aldehyde.

The polymerization initiator used in the polymerization step is not particularly limited, and can be selected from known polymerization initiators depending on the polymerization method. Examples of the polymerization initiator include an azo-based polymerization initiator, a peroxide-based polymerization initiator, a redox-based polymerization initiator, and the like. Examples of the azo-based polymerization initiator include 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (isobutyronitrile), and 2,2'-azobis (4-methoxy-). 2,4-Dimethylvaleronitrile) and the like. The peroxide-based polymerization initiator is, for example, a percarbonate-based compound such as diisopropyl peroxy dicarbonate, di (2-ethylhexyl) peroxy dicarbonate, diethoxyethyl peroxy dicarbonate; t-butyl peroxyneodecano. Perester compounds such as ate and cumylperoxyneodecanoate; acetylcyclohexylsulfonyl peroxide; 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate and the like. As the redox-based polymerization initiator, a combination of an oxidizing agent and a reducing agent can be used. Peroxide is preferable as the oxidizing agent. Examples of the reducing agent include metal ions and reducing compounds. Examples of the combination of the oxidizing agent and the reducing agent include a combination of a peroxide and a metal ion; a combination of a peroxide and a reducing compound; and a combination of a peroxide and a metal ion and a reducing compound. Examples of the peroxide include hydrogen peroxide, hydroxyperoxide such as cumene hydroxyperoxide and t-butyl hydroxyperoxide, persulfate (potassium, sodium or ammonium salt), t-butyl peracetate, and peracid ester (peracid). T-butyl benzoate) and the like. Examples of the metal ion include metal ions capable of receiving one electron transfer such as Fe ²⁺ , Cr ²⁺ , V ²⁺ , Co ²⁺ , Ti ³⁺ , and Cu ⁺ . Examples of the reducing compound include sodium bisulfite, sodium hydrogen carbonate, tartaric acid, fructose, dextrose, sorbose, inositol, longalite and ascorbic acid. Among these, one or more peroxides selected from the group consisting of hydrogen peroxide, potassium persulfate, sodium persulfate and ammonium persulfite, and sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, longalite and ascorbic acid. A combination with one or more reducing agents selected from the group is preferable, and hydrogen peroxide and one or more reducing agents selected from the group consisting of sodium hydrogen sulfite, sodium hydrogen carbonate, tartrate acid, longalite and ascorbic acid. The combination of is more preferable. Further, a water-soluble polymerization initiator such as potassium persulfate, ammonium persulfate, hydrogen peroxide, and cumene hydroperoxide may be combined with the above-mentioned polymerization initiator to obtain a polymerization initiator. These polymerization initiators may be used alone or in combination of two or more.

When the vinyl ester is polymerized in the presence of dialdehyde or trialdehyde, the polymerization rate of the vinyl ester is not particularly limited, but is preferably 10% or more and less than 90%. If the polymerization rate is less than 10%, the productivity of PVA may decrease. The polymerization rate is more preferably 20% or more. On the other hand, when the polymerization rate is 90% or more, there is a possibility that the viscosity of the obtained polyvinyl ester becomes too high, the productivity of PVA decreases, and the hue of the obtained PVA deteriorates. The polymerization rate is more preferably less than 70%.

The method for saponifying the polyvinyl ester is not particularly limited, and a known saponification method can be adopted. For example, an alcohol decomposition reaction or a hydrolysis reaction using a basic catalyst such as sodium hydroxide, potassium hydroxide or sodium methoxide or an acidic catalyst such as p-toluenesulfonic acid can be mentioned. Examples of the solvent that can be used in this reaction include alcohols such as methanol and ethanol; esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone: aromatic hydrocarbons such as benzene and toluene. One of these solvents may be used alone, or two or more of them may be used in combination. Above all, a method of saponifying using methanol or a mixed solution of methanol / methyl acetate as a solvent and sodium hydroxide as a catalyst is convenient and preferable.

(Use)
The PVA of the present invention is used for various purposes. Examples are given below, but the present invention is not limited to this.
(1) Dispersant Applications: Dispersion stabilizers for pigments such as paints and adhesives, dispersion stabilizers for suspension polymerization of various vinyl compounds such as vinyl chloride, vinylidene chloride, styrene, (meth) acrylate, and vinyl acetate, and dispersion aids. Agent (2) Coating agent Application: Paper coating agent, sizing agent, fiber processing agent, leather finishing agent, paint, antifogging agent, metal corrosion inhibitor, zinc plating brightener, antistatic agent, pharmaceutical coating agent (3) ) Adhesive applications: Adhesives, adhesives, re-wet adhesives, various binders, additives for cement and mortar (4) Emulsifier applications: Emulsifiers for emulsification and polymerization, post-embroidery such as bitumen (5) Coagulant applications: Suspended in water Coagulant for turbid and dissolved substances, metal coagulant (6) Paper processing applications: Paper strength enhancer, oil / solvent resistance imparting agent, smoothness improver, surface gloss improving aid, sealant, barrier agent, light resistance Sex-imparting agent, water resistant agent, dye / color developer dispersant, adhesive strength improver, binder (7) Agricultural use: Agricultural chemical binder, agricultural chemical spreading agent, agricultural coating agent, soil improver, erosion inhibitor , Dispersants for pesticides (8) Medical / cosmetic applications: Granulation binders, coating agents, emulsifiers, patches, binders, film formulation base materials, film forming agents (9) Viscosity adjusters Applications: thickeners, rheology adjustment Agent (10) Film Applications: Water-soluble films, polarizing films, barrier films, textile packaging films, seed curing sheets, vegetation sheets, seed tapes, moisture-absorbing films (11) Molded products Applications: Fibers, pipes, tubes, proof Water-soluble fiber for leak film, chemical race, sponge (12) Gel Application: Pharmaceutical gel, Industrial gel (13) Post-reaction application: Post-reaction application with low molecular weight organic compound, high molecular weight organic compound, inorganic compound By using an acid catalyst, PVA constrains the movement of the molecular chain in the three-dimensional direction, and has higher viscosity, higher water resistance, higher strength, heat resistance, and solvent resistance than linear polymers of the same type. Can synthesize the body. Therefore, it is suitably used for the above-mentioned (2) coating material use, (3) adhesive use, (10) film use, (12) gel use and the like. Further, the PVA of the present invention is suitably used for (1) dispersant use as described later.

(Dispersion stabilizer for suspension polymerization of vinyl compounds)
A suitable use of the PVA of the present invention is a dispersion stabilizer for suspension polymerization of a vinyl compound containing the PVA. When the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization reaction is stabilized and the formation of coarse particles and fine powder can be suppressed. In addition, it is possible to suppress the occurrence of fish eyes when the obtained vinyl resin is made into a sheet.

The dispersion stabilizer for suspension polymerization may contain various additives as long as the gist of the present invention is not impaired. Examples of the additive include polymerization inhibitors such as aldehydes, halogenated hydrocarbons and mercaptans; polymerization inhibitors such as phenol compounds, sulfur compounds and N-oxide compounds; pH adjusters; cross-linking agents; preservatives. Examples include antifungal agents; antiblocking agents; antifoaming agents; compatibilizers and the like. The content of various additives in the dispersion stabilizer for suspension polymerization is preferably 10% by mass or less, more preferably 5% by mass or less, based on the total amount of the dispersion stabilizer for suspension polymerization.

(Manufacturing method of vinyl resin)
Another preferred embodiment of the present invention is a method for producing a vinyl resin in which a vinyl compound is suspended and polymerized in the presence of PVA of the present invention. In such a production method, a particulate vinyl resin can be obtained.

Examples of the method of charging the dispersion stabilizer for suspension polymerization of the present invention into the polymerization tank include (i) a method of preparing an aqueous solution and charging it into the polymerization tank, and (ii) a method of charging it in a powder state. From the viewpoint of uniformity in the polymerization tank, the method (i) above is preferable.

Examples of vinyl compounds include vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acids, methacrylic acids, esters and salts thereof; maleic acid, fumaric acid, esters and anhydrides thereof; styrene. , Acrylonitrile, vinylidene chloride, vinyl ether and the like. Of these, it is preferable to use vinyl chloride alone, or to use vinyl chloride and a monomer copolymerizable with vinyl chloride in combination. Examples of the monomer copolymerizable with vinyl chloride include vinyl esters such as vinyl acetate and vinyl propionate; (meth) acrylate esters such as methyl (meth) acrylate and ethyl (meth) acrylate; ethylene and propylene. α-Olefin; unsaturated dicarboxylic acids such as maleic anhydride and itaconic acid; acrylonitrile, styrene, vinylidene chloride, vinyl ether and the like.

For suspension polymerization of the vinyl compound, an oil-soluble or water-soluble polymerization initiator conventionally used for polymerization of vinyl chloride can be used. Examples of the oil-soluble polymerization initiator include percarbonate compounds such as diisopropylperoxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethylperoxydicarbonate; t-butylperoxyneodecanoate, t. Perester compounds such as −butylperoxypivalate, t-hexylperoxypivalate, cumylperoxyneodecanoate; acetylcyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate, Peroxides such as 3,5,5-trimethylhexanoyl peroxide, lauroyl peroxide, etc .; 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (isobutyronitrile), Examples thereof include azo compounds such as 2,2'-azobis (4-methoxy-2,4-dimethylvaleronitrile). Examples of the water-soluble polymerization initiator include potassium persulfate, ammonium persulfate, hydrogen peroxide, cumene hydroperoxide and the like. One of these polymerization initiators may be used alone, or two or more thereof may be used in combination.

In the suspension polymerization of the vinyl compound, the polymerization temperature is not particularly limited, and may be a low temperature of about 20 ° C. or a high temperature of more than 90 ° C., and more preferably about 20 to 60 ° C. Further, in order to increase the heat removal efficiency of the polymerization reaction system, a polymer with a reflux condenser may be used.

The obtained vinyl-based resin can be used for various molded products by appropriately blending a plasticizer or the like.

In the suspension polymerization of a vinyl compound, the amount (concentration) of the dispersion stabilizer for suspension polymerization of the present invention is usually 1000 ppm or less and 50 ppm or more with respect to the vinyl compound. If it is less than 50 ppm, coarse particles may easily be generated during suspension polymerization of the vinyl compound. The ppm means mass ppm.

In addition to PVA during suspension polymerization of vinyl compounds, water-soluble cellulose ethers such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and hydroxypropyl methyl cellulose, which are usually used for suspension polymerization of vinyl compounds in an aqueous medium; gelatin. Water-soluble polymers such as; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan triolate, glycerin tristearate, ethylene oxide propylene oxide block copolymer; polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate, sodium laurate, etc. A water-soluble emulsifier or the like may be used in combination. The amount added is not particularly limited, and is preferably 0.01 part by mass or more and 1.0 part by mass or less per 100 parts by mass of the vinyl compound.

(Crosslink)
A suitable use of the PVA of the present invention is a crosslinked product in which the PVA is crosslinked, and the dissolution rate when a film having a thickness of 100 μm made of the crosslinked product is immersed in hot water at 80 ° C. for 1 hour is less than 10%. It is a crosslinked body.

Here, the elution rate can be measured by the method described in Examples described later, and it can be said that the smaller the elution rate, the better the water resistance. Further, the film to be measured is produced by a known method such as a casting film forming method or a melt extrusion film forming method using a film forming stock solution containing a crosslinked product produced by using the PVA of the present invention. be able to. In the present invention, the elution rate when the film is immersed in hot water at 80 ° C. for 1 hour is more preferably less than 8%, and even more preferably less than 5%.

The method for producing the crosslinked product is not particularly limited, but a method for producing the PVA in the presence of an acid catalyst is preferable.

(Acid catalyst)
The type of acid catalyst is not particularly limited, and examples thereof include phosphoric acid, hydrochloric acid, and sulfuric acid. Among them, phosphoric acid is preferably used.

The method of using the acid catalyst is not particularly limited. The acid catalyst may be used as it is, or the acid catalyst may be dissolved in a solvent and used. Further, when mixing with PVA, the acid catalyst may be mixed after preparing the PVA aqueous solution, or the acid catalyst may be mixed and dissolved at the same time as preparing the PVA aqueous solution. A method of mixing an acid catalyst after preparing an aqueous PVA solution is preferable from the viewpoint of suppressing side reactions.

The amount of the acid catalyst used is not particularly limited, but the amount of the acid catalyst used is preferably 0.01 parts by mass or more and 5 parts by mass or less with respect to 10 parts by mass of PVA. If the amount used is less than 0.01 parts by mass, the crosslinked body may not be formed well. The amount used is more preferably 0.05 parts by mass or more. On the other hand, if the amount used exceeds 5 parts by mass, the relative concentration of PVA decreases, so that the crosslinked body may not be formed well. The amount used is more preferably 3 parts by mass or less.

In the production of the crosslinked product, a crosslinking agent can be used in combination with the above acid catalyst. Examples of cross-linking agents include dialdehydes such as glyoxal, malondialdehyde and glutaaldehyde, glyoxyphosphates such as sodium glyoxylate and calcium glyoxylate, diamines such as ethanediamine, propanediamine and 1,3-bisaminomethylcyclohexane. Examples include dihydrazides such as adipic acid dihydrazide.

The method of using the cross-linking agent is not particularly limited, and the same method as that of the acid catalyst can be used. Further, the amount of the cross-linking agent used is not particularly limited, and can be the same as the amount of the acid catalyst used.

[Viscosity average degree of polymerization of PVA]
The viscosity average degree of polymerization of PVA was measured according to JIS K 6726: 1994. Specifically, when the degree of saponification of PVA is less than 99.5 mol%, the ultimate viscosity [η] (η) measured at 30 ° C. in water for PVA saponified to a degree of saponification of 99.5 mol% or more. The viscosity average degree of polymerization (P) was determined by the following formula using L / g).
P = ([η] × 10 ^{4 /} 8.29) ^{(1 / 0.62)}

[Saponification degree of PVA]
The degree of saponification of PVA was measured according to JIS K 6726: 1994.

[Content of terminal aldehyde group of PVA]
For the content of the terminal aldehyde group of PVA, a 10% by mass aqueous solution of PVA was prepared, and 5 g of this aqueous solution was added dropwise to a solution of 500 g of methyl acetate / water = 95/5 to precipitate PVA, which was recovered and dried. , The isolated PVA is dissolved in DMSO-d ₆ and measured using ¹ H-NMR at 400 MHz. At this time, the peak derived from methine of the vinyl alcohol unit is assigned to around 3.2 to 4.0 ppm (integral value P), and the peak of the proton derived from the aldehyde group is attributed to around 9.5 to 10 ppm (integral value Q). Then, the content was calculated from each peak by the following formula.
Aldehyde group content (mol%) = (Q / P) x 100

[Absorptivity by UV absorption spectrum of PVA]
A 0.1% by mass aqueous solution of PVA was prepared. Then, the aqueous solution was placed in a cell having an optical path length of 1 cm, and the absorbance at 280 nm was measured using an ultraviolet-visible spectrophotometer (UV-2450 manufactured by Shimadzu Corporation).

[Manufacturing Example 1 Production of PVA1]
1500 parts by mass of vinyl acetate (hereinafter, may be abbreviated as "VAc") and 10 parts by mass of methanol were charged into the polymerization tank. Next, after replacing the inside of the polymerization tank with nitrogen, 12 parts by mass of glutaraldehyde and 12 parts by mass of water were added to the polymerization tank, and the polymerization tank was heated to raise the temperature to 60 ° C., and 2,2'-as a polymerization initiator. Polymerization was carried out in the presence of azobis (isobutyronitrile) until the polymerization rate became 25%. The operation of expelling the remaining VAc under reduced pressure to the outside of the system together with methanol was performed while adding methanol to obtain a methanol solution (concentration: 40% by mass) of polyvinyl acetate (hereinafter, may be abbreviated as "PVAc"). Next, in a methanol solvent, the concentration of vinyl acetate units in PVAc was diluted to 30% by mass, and saponification was performed for 1 hour using sodium hydroxide at a temperature of 40 ° C. and a molar ratio of 0.03 to PVAc as a saponification catalyst. The reaction was carried out. The obtained polyvinyl alcohol was immersed in a washing solution of methyl acetate / methanol = 80/20 for washing. Then, the solvent was removed by centrifugation and then dried, and the viscosity average degree of polymerization was 1500, the saponification degree was 99 mol%, the terminal aldehyde group content was 0.1 mol%, and 0.1 mass. A PVA1 having an absorbance of 0.279% aqueous solution at 280 nm was obtained.

[Production Examples 2 to 12 (Production of PVA 2 to 12)]
PVA2 is the same as in Production Example 1 except that the amounts of vinyl acetate and methanol, water, and aldehyde used during polymerization, the type of aldehyde, the polymerization rate when the polymerization reaction is stopped, and the saponification conditions are changed as shown in Table 1. ~ PVA12 was produced. The production conditions are shown in Table 1, and the types of aldehydes used are shown in Table 2.

[Manufacturing Example 13 (Manufacturing of PVA13)]
PVA11 was produced in the same manner as in Production Example 11, and the obtained PVA11 was heat-treated at 80 ° C. for 1 hour in a hot air dryer to produce PVA13.

Example 1
10 parts by mass of PVA (1) was dissolved in distilled water to prepare 100 parts by mass of a 10% by mass aqueous solution, 0.5 part by mass of phosphoric acid was added as an acid catalyst, and the mixture was mixed and stirred to prepare an aqueous solution of a resin composition. Such an aqueous solution was cast on a polyethylene terephthalate (PET) film, left at 23 ° C. and 50% RH for 48 hours, and then heat-treated at 70 ° C. for 5 minutes to obtain a film having a thickness of 100 μm. When the water resistance of the obtained film was evaluated as follows, the dissolution rate was 1.5% by mass.

(water resistant)
The obtained film was immersed in hot water at 80 ° C. for 1 hour, and the elution rate (mass%) of the film was measured. In calculating the elution rate (mass%), the dry mass X1 (g) of the film before immersion in hot water and the dry mass X2 (g) of the film after immersion in hot water were obtained, and the elution rate (mass%) was calculated by the following formula. Mass%) was calculated. The results are shown in Table 3.
Elution rate (mass%) = [(X1-X2) / X1] x 100

Example 2
The water resistance was evaluated in the same manner as in Example 1 except that the type of PVA used was changed. The results are shown in Table 3.

Comparative Example 1
A film was prepared in the same manner as in Example 1 except that PVA6 was used as PVA, and evaluation of water resistance was attempted. As a result, PVA6 had too low an aldehyde group content, so that water resistance was hardly exhibited.

Comparative Example 2
An attempt was made to evaluate the water resistance in the same manner as in Example 1 except that PVA7 was used as PVA. As a result, PVA7 had almost no water resistance because the viscosity average degree of polymerization was too low.

Example 3
PVA3 was dissolved in deionized water as a dispersion stabilizer for suspension polymerization, and 100 parts by mass of an aqueous solution of PVA3 was charged into an autoclave. The amount of PVA3 charged was 850 ppm with respect to the amount of vinyl chloride (VCM) charged. Then, the deionized water was added so that the total amount of the deionized water was 1200 parts by mass. Next, 0.65 parts by mass of a 70% by mass toluene solution of cumylperoxyneodecanoate and 1.05 parts by mass of a 70% by mass toluene solution of t-butylperoxyneodecanoate were added to the autoclave and placed in the autoclave. Toluene was introduced so that the pressure was 0.2 MPa. After that, the operation of purging nitrogen was performed a total of 5 times, and the inside of the autoclave was sufficiently replaced with nitrogen to remove oxygen, and then 940 parts by mass of vinyl chloride was added. The contents in the autoclave were heated to 57 ° C. and suspension polymerization of vinyl chloride was started under stirring. The pressure in the autoclave at the start of polymerization was 0.80 MPa. Approximately 3.5 hours after the start of the polymerization, the polymerization was stopped when the pressure in the autoclave reached 0.70 MPa, unreacted vinyl chloride was removed, and the polymerization reaction product was taken out and at 65 ° C. 16 It was dried for a time to obtain vinyl chloride polymer particles. Then, the vinyl chloride polymer particles obtained by the method shown below were evaluated.

(Evaluation of vinyl chloride polymer particles)
With respect to the obtained vinyl chloride polymer particles, (1) average particle size, (2) particle size distribution and (3) fish eye were evaluated according to the following methods. The evaluation results are shown in Table 4.

(1) Average particle size Using a sieve based on the Tyler mesh, the particle size distribution was measured by the dry sieve method described in JIS Z 8815: 1994. The results were plotted in a Rosin-Rammler distribution formula to calculate the average particle size (d _p50 ).

(2) Particle size distribution The content (mass%) of vinyl chloride polymer particles that did not pass through a sieve with a mesh size of 355 μm (42 mesh in terms of mesh of JIS standard sieve) was evaluated according to the following evaluation criteria. The content means cumulative on a sieve (mass%). Further, the opening of the sieve conforms to the nominal opening W of JIS Z 8801-1-2006.
A: Less than 0.5% by mass B: 0.5% by mass or more and less than 1% by mass C: 1% by mass or more

The content (mass%) of the vinyl chloride polymer particles that passed through a sieve with a mesh size of 355 μm and did not pass through a sieve with a mesh size of 250 μm (60 meshes in terms of JIS standard mesh) was evaluated according to the following evaluation criteria. The content means cumulative on a sieve (mass%). Further, the opening of the sieve conforms to the nominal opening W of JIS Z 8801-1-2006.
A: Less than 5% by mass B: 5% by mass or more and less than 10% by mass C: 10% by mass or more

The content (mass%) of the vinyl chloride polymer particles that passed through a sieve having a mesh size of 75 μm (200 mesh in terms of JIS standard mesh) was evaluated according to the following evaluation criteria. The content means cumulative on a sieve (mass%). Further, the opening of the sieve conforms to the nominal opening W of JIS Z 8801-1-2006.
A: Less than 1% by mass B: 1% by mass or more and less than 2% by mass C: 2% by mass or more

The smaller the value of both the content of the vinyl chloride polymer particles that did not pass through the sieve with a mesh size of 355 μm and the content of the vinyl chloride polymer particles that did not pass through the sieve with a mesh size of 250 μm, the smaller the coarse particles. It shows that the particle size distribution is sharp and the polymerization stability is excellent. Further, the fact that the content of the vinyl chloride polymer particles that have passed through the 75 μm sieve is small indicates that the amount of fine powder is small and the process passability is excellent.

(3) Fisheye 100 parts by mass of the obtained vinyl chloride polymer particles, 50 parts by mass of dioctyl phthalate, 5 parts by mass of lead tribasic lead sulfate and 1 part by mass of zinc stearate are roll-kneaded at 150 ° C. for 7 minutes. A 1 mm thick sheet was prepared and the number of fish eyes per 1000 cm ² was visually measured. The smaller the number of fish eyes, the fewer defects on the sheet.

Examples 4-5
Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3 except that PVA4 to 5 were used instead of PVA3. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. When the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization, the obtained vinyl chloride polymer particle size does not become coarse, good polymerization stability is exhibited, fine powder is small, and the number of fish eyes is small. There were few.

Comparative Example 3
Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3 except that PVA8 was used as PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since PVA8 has a large content of aldehyde groups, the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine particles is large, and the number of fish eyes is also large.

Comparative Example 4
Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3 except that PVA9 was used as PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since the degree of saponification of PVA9 is too low, the average particle size of the obtained vinyl chloride polymer particles is large, the proportions of coarse particles and fine particles are large, and the number of fish eyes is also large.

Comparative Example 5
Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3 except that PVA10 was used as PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since the viscosity average degree of polymerization of PVA10 is too high, the average particle size of the obtained vinyl chloride polymer particles is large, the proportions of coarse particles and fine particles are large, and the number of fish eyes is also large.

Comparative Example 6
Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3 except that PVA11 was used as PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Since the content of the aldehyde group in PVA11 is too small, the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine particles is large, and the number of fish eyes is also large.

Comparative Example 7
Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3 except that PVA12 was used as PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. PVA12 is a PVA derived from polyvinyl acetate obtained by polymerizing vinyl acetate in the presence of monoaldehyde, and since it does not have an aldehyde group at the end of PVA, the average particle size of the obtained vinyl chloride polymer particles is large. The result was that it was large, had a large proportion of coarse particles and fine powder, and had a large number of fish eyes.

Comparative Example 8
Suspension polymerization of vinyl chloride was carried out in the same manner as in Example 3 except that PVA13 was used as PVA. The evaluation results of the obtained vinyl chloride polymer particles are shown in Table 4. Although PVA13 has a high absorbance value, the content of aldehyde groups is too small, so that the average particle size of the obtained vinyl chloride polymer particles is large, the proportion of coarse particles and fine particles is large, and the number of fish eyes is also large. ..

As shown in Examples, the PVA of the present invention was able to easily form a crosslinked product using an acid or the like, and was excellent in water resistance at that time. Further, when the PVA of the present invention is used as a dispersion stabilizer for suspension polymerization of a vinyl compound, the polymerization stability is excellent, and the obtained vinyl chloride polymer particles (vinyl-based polymer) have a small average particle size, and are coarse particles and fine powder. Is less likely to occur, and fish eyes can be reduced. Therefore, the productivity and processability of the vinyl polymer are excellent. Therefore, the industrial usefulness of the present invention is extremely high.

Claims (8)

The degree of saponification is 70 mol% or more and less than 99.9 mol%, and the viscosity average degree of polymerization is 400 or more and less than 1800.
A polyvinyl alcohol containing 0.05 mol% or more and less than 0.5 mol% of an aldehyde group at the terminal, and having an absorbance of a 0.1 mass% aqueous solution at 280 nm of 0.17 or more and less than 0.55. The polyvinyl alcohol according to claim 1, which has a structure represented by the following formula (1) at the end.

(In the formula (1), X represents a single bond, an alkylene group which may have a substituent or an arylene group which may have a substituent, and * represents a bond.) The polyvinyl alcohol according to claim 2, wherein X is an alkylene group having 1 to 6 carbon atoms. The method for producing polyvinyl alcohol according to any one of claims 1 to 3, wherein the vinyl ester is polymerized in the presence of dialdehyde or trialdehyde to obtain a polyvinyl ester, and then the polyvinyl ester is saponified. A dispersion stabilizer for suspension polymerization of a vinyl compound containing the polyvinyl alcohol according to any one of claims 1 to 3. A method for producing a vinyl resin in which a vinyl compound is suspended and polymerized in the presence of polyvinyl alcohol according to any one of claims 1 to 3. The crosslinked product in which the polyvinyl alcohol according to any one of claims 1 to 3 is crosslinked, and the elution rate when a film having a thickness of 100 μm made of the crosslinked product is immersed in hot water at 80 ° C. for 1 hour is 10%. A crosslinked body that is less than. The method for producing a crosslinked product according to claim 7, wherein the polyvinyl alcohol is crosslinked in the presence of an acid catalyst.